From the beginning of semiconductor device technology, physicists have postulated a host of semiconductor heterostructures, i.e., combinations of layers of different semiconductor materials in a single composite crystal. The fascination in this kind of structure is traceable to the nearly infinite selection of electrical properties thus potentially available to device designers. The early visions of a wide variety of hetero combinations were never realized. It is now recognized that semiconductor heterostructures with perfection suitable for state of the art devices are extremely difficult to produce. An approach that has proved successful, and is widely used, is that of combining III-V or II-VI semiconductors with ternary materials in the same system, e.g., GaAs and GaAlAs. Layers with slightly different amounts of the ternary addition can be graded to reduce the effect of the compositional discontinuity. Such structures are used widely in optoelectronic devices.
To date, all of the heterostructures that have met with technological success have been combinations of semiconductor materials.